Study On Determining Representative Elementary Volume Of Equivalent Rock Mass And The Mechanical Behavior Of Joints

The joints widely existing in the engineering of rock mass have so significant effect on the mechanical behavior of rock mass that the rock mass is characterized by discontinuity, heterogeneity and anisotropy. At the same time, the size effect of the mechanical parameters of rock mass is caused by the joint. The properties of jointed rock mass are complex, and it is the key and difficult point in the research of rock mechanics. On the engineering background of Baiyunbo Eastern Iron Open-pit in Inner Mongolia and Pingshuo Open Pit Coal Mine in Shanxi Province, field joint test line investigation and laboratory mechanical tests are conducted, and based on the principle of Monte Carlo stochastic simulation and block discrete element theory equivalent jointed rock masses models are set up. A series of studies on the mechanical properties of jointed rock mass are conducted from the static and dynamic two aspects, which provide a theoretical basis for the assessment of stability and safety in rock mass engineering and serve the rapid development of rock engineering construction.1) Static properties of jointed rock massThe studies on static properties of jointed rock mass are conducted using Coulomb slip joint model. The studies include:equivalent jointed rock mass models with stochastic joints are set up combining three dimensional network simulation technology and block discrete element method; uniaxial and triaxial compression tests of different scales equivalent rock mass are conducted, and the research on representative elemental volume (REV) of the mechanical parameters are conducted. The main research results are as follows:(1) At begin with, joints samples are collected by way of a scanline method, the probability statistics, the error correction and the parameter estimation are carried out, the probability distribution model and parameters are determined and random geological parameters data are obtained using the Monte-Carlo stochastic simulation theory. Secondly, by FISH language programming the 3D network model of rock mass is generated using random data and penetration rate of joint. Finally, using Coulomb slip joint model and the strain softening rock model parameters of which are determined from laboratory tests, equivalent joint rock mass with different size considering the mechanical characteristics after peak is established by 3 dimensional distinct element code(3DEC).By means of the stress-strain curves obtained from the uniaxial and triaxial compression test, the equivalent jointed rock mass set up using this method can reflect the brittle properties of rock and the elastoplastic properties of rock mass.(2) Uniaxial compression test program is compiled by FISH language. The uniaxial compression tests of the equivalent rock mass show that the stress-strain curves of the rock mass is divided into five stages:elastic-joint slipping-elastoplastic-softening-residual strength; that the peak strength, deformation modulus and residual strength decrease gradually with the increase of the equivalent rock mass scale, and when the object’s size increases to 15m-20m, they decrease to constants, so the REV of rock mass, the peak strength, deformation modulus of REV are obtained.(3) Triaxial compression test program is compiled by FISH language. The triaxial compression tests of the equivalent rock mass show that the triaxial compressive strength under the same scale increases with the confining pressure, but the its increasing amplitude and the amplitude of the confining pressure have nonlinear relationship(the its increasing amplitude decrease gradually); that unde the same confining pressure, influence of confining pressure on mechanical behavior of rock mass increase with the increase of rock mass scale and number of joint; that REV has a decreasing trend with the increase of confining pressure, and the confining pressure has less effect on the REV comparing the uniaxial compression.2) Dynamic properties of jointed rock massCoulomb slip joint model is suitable for the static analysis of joints. However, the joints in rock engineering are often under the action of blasting load, and the influence of dynamic load on REV of the equivalent rock mass is still unknown. Therefore, in this paper the basic researches on continuous yield joint model that can realize cyclic shearing are carried out, which lay the foundation for the study on REV of dynamic parameters and dynamic response analysis of the rock mass. The studies include:the mechanical parameters of continuous yield joint are determined, the properties of joints under normal cyclic loading/unloading and under cyclic shearing are researched. The results obtained are as follows:(1) A method for determining the mechanical parameters of the continuous yield joint model is given according to the laboratory test of the joints. Continuous yield joint model is used to carry out the joint direct shear numerical experiment, and the shear stress-displacement curves are obtained. The curves include 4 stages: increasing-peak-strain softening-residual strength. Comparison of indoor test and numerical experiment shows that continuously yielding joint model can better reflect the nonlinear relationship between the shear stress and the shear deformation of the joint, and can reflect shear peak strength, strain softening and residual strength after shear peak strength.(2) The parameters such as the joint surface roughness, the difference of the initial friction angle and the basic friction angle are studied. The research shows that:① under the same shear stiffness condition, the influence of the roughness on the shear stress is mainly observed in the peak strength, strain softening and residual strength stage, and the shear peak strength increases exponentially with the increase of the roughness.② The peak strength of the joint is linear correlation with the difference of the initial friction angle and the basic friction angle, and the slope of the strain softening stage in the shear stress-displacement curve is increased with the increase of the friction angle difference.(3) Semi-logarithmic constitutive equation and hyperbolic constitutive equation are respectively used to fit joint normal stiffness-closure curves obtained from the joint normal cyclic loading/unloading experiment, and joint normal stiffness expressions are gained from two kinds of constitutive equations. These expressions are implemented into continuously yielding joint model, and cyclic loading/unloading programs are edited by FISH language embedded within 3 dimensional distinct element code(3DEC). The numerical simulation of a rock joint under normal cyclic loading/unloading is conducted. Comparison of indoor test and numerical experiment shows that the numerical simulation method in this paper is valid, that the change trend of two kinds of joint normal stiffness values is similar with normal stess increasing, and that each loading/unloading cycle, there is a hysteresis loop and the energy dissipation, with the increase of number of cycles, the area of hysteresis loop reduce and jointed energy dissipation decreases.(4) Based on continuously yielding joint model, the cyclic shear tests of the joints under different normal stress and shear displacement are carried out. Studies on influence laws of cyclic number, normal stress and cyclic shear displacement on the joint cyclic shear mechanical parameters (cyclic shear strength, dynamic shear modulus and area of hysteresis loop) are conducted and fitting influence relationship is obtained.Hysteretic loop curves of joints under cyclic shearing show that with the increase of cyclic shear displacement, hysteresis loop area under the same stress increases (mainly to an increase in the width of the hysteresis loop) and the joint energy consumption increases; with the normal stress increase, hysteresis loop area under the same stress displacement (mainly in increasing hysteresis loop height) and the joint energy consumption increases.The influence of cyclic number and cyclic shear displacement on the decrease of cyclic shear strength (the increasing of shear strength damage factor) is obvious before peak value of shear stress, but is not obvious after peak value of shear stress. Joint dynamic shear modulus and area of hysteresis loop are influenced slightly by cyclic number, but affected significantly. In this paper, the influence relationship of the cyclic shear displacement and normal stress on cyclic shear mechanical parameters is fitted by the exponential equation.To sum up, the main original research and application in three aspects are conducted:① Using Coulomb sliding joint model, equivalent rock mass models with stochastic joints 3D-network are setup by the block discrete element software 3DEC; uniaxial and triaxial compression tests for equivalent rock mass are conducted; the REV of the compressive strength and deformation modulus in Baiyunbo Eastern Iron Open-pit in Inner Mongolia is determine.② A method for determining the mechanical parameters of continuously yield joint model is given. A new method based on the nonlinear normal deformation of the joints is presented, which makes up the defects of the continuously yield joint model in the indication of the deformation characteristics. The joint stiffness equation based on the hyperbolic constitutive equation is introduced into the continuously yield joint model, and the model is improved and developed.③ Based on continuously yield joint model, joints cyclic loading numerical test are carried out. The influence laws of cyclic number, normal stress and cyclic shear displacement on the joint cyclic shear mechanical parameters (cyclic shear strength, dynamic shear modulus and area of hysteresis loop) are conducted and fitting influence relationship is obtained by the exponential equation.